Capacitive probe



Oct. 26, 1965 s. F. DI GIACOMO 3,214,655

CAPACITIVE PROBE Filed May 29. 1962 H fl INVENTOR. 5' 1 555mm fD/hcomo 73 BY did 1/ My 5 ATToBA/Er United States Patent 3,214,655 CAPACITIVE PROBE Sebastian F. Di Giacomo, Brooklyn, N.Y., assignor to The Liquidometer Corp., Long Island City, N.Y., a corporation of Delaware Filed May 29, 1962, Ser. No. 198,616 6 Claims. (Cl. 317-246) The present invention relates to a capacitive probe which is arranged to be used in measuring the amount of liquid in irregularly-shaped tanks, so constructed and arranged that the change in capacitance shall be proportional to the liquid contents throughout the portions of the tank being measured; in other words, so that one micromicrofarad of capacitance shall be equivalent to a definite number of gallons or pounds of liquid being measured irrespective'of the shape of the tank or portions thereof.

The capacitive probe of the present invention is particularly designed for use in tanks holding cryogenic liquids, for example, liquified gases such as hydrogen, oxygen, etc.

In the development of modern missiles it has often been found desirable to use liquid fuels and liquid oxygen as energy sources, the fuels being, for example, hydrogen or possibly very light hydrocarbons and the oxygen sources including oxygen per se, hydrogen peroxide and possibly others. Some missiles, for example, are designed to use a hydrogen fuel and oxygen as the material to combine with the fuel, at least in one stage. In the construction of one such missile, it has been found con venient to have a tank for liquid hydrogen occupying a substantial part of the entire cross-section of the missile, advantageously in a cylindrical form with a spherical liquid oxygen tank having substantially the same diameter as the diameter of the cylindrical portion of the hydrogen tank and forming the bottom thereof, so that the two are tangent at a circular line. In such an arrangement it has been found necessary that means be provided for determining at each and every instant the amount of hydrogen present in the hydrogen tank. This data along with corresponding data as to the amount of oxygen present in the oxygen tank can then be fed to a computer carried in the missile, which controls the rate of flow of these two gases to a combustion point. At the moment when both gases have been completely used (which should be simultaneous), means may be actuated for separating this section of the missile from any section or sections thereof which are to proceed further. In order that the data be supplied in a form in which it can be properly used by the computer in a device of this kind, it is necessary that the information as to the amount of fuel present at any one instant be substantially a linear function of that amount of fuel (considering hydrogen as fuel). The probe of the present invention, while capable of other uses, is peculiarly adapted to the determination of the amount of fuel (hydrogen) in a missile of this kind and to provide data which is usable in accordance with the principles set out hereinabove.

Capacitive probes and capacitance responsive electrical systems for use therewith have now come into general use in measuring the fuel used in aircraft and may be said to be well known. For example, one such system using a capacitive probe and capable of indicating the quantity of a liquid being measured in terms of either weight or volume (selectively) is described and claimed in detail in the patent to Campani, No. 2,738,673, granted March 20, 1956.

It has also been proposed and, to a certain extent, has become common practice, to profile capacitive probes so as to compensate for variations in the cross-sectional areas of tanks being measured. Such profiling has taken 3,214,655 Patented Oct. 26, 1965 the form of a variation in the diameter of the inner elec trode in installations where the probe is generally in the form of concentric inner and outer electrodes with the liquid to be measured admitted to the annular space there between. In general, as the cross-sectional area available for liquid storage at a given level is reduced from a normal standard (for the container being measured)", the inner electrode is made of progressively smaller diameter. This is quite adequate in most instances to accommodate the variations normally encountered. It was found, however, in measuring the amount of liquid in a space corre sponding in size and character to that between a spherical surface and a cylindrical surface, wherein the spherical surface extends inwardly from one end of the cylinder and is concentric therewith and tangent thereto, and with the probe extending generally into this narrow portion of the liquid space, the requirements for attaining proportionality between the variations in capacitance and in liquid contents by generally cutting down the diameter of the inner electrode resulted in an impossible physical structure, as the diameter of the inner electrode worked out mathematically to be only a few thousandths of an inch. If the same result of lowering capacitance change per unit of vertical height were to be obtained by enlarging the diameter of the outer electrode at this point, then the probe would be so large in diameter that it would not fit down into the narrow space to be measured and there would be a substantial dead area. Either of these expedients were, therefore, automatically rejected as practically impossible of attainment.

In view of the fact that a cryogenic liquid was to be measured, certain other limitations had to be observed in order that the device should have adequate strength and serviceability under the extremely low temperature and high pressure conditions in which it must operate.

The present invention is a practical solution of all these difficulties in that it employs standard and unchanged diameters and hence undiminished strengths for both the inner and outer electrodes, and interposes therebetween an amount of an insulating material, which is preferably in the form of a sleeve closely surrounding the inner electrode, and having a thickness calculated in accordance with the free area of the container for liquid at each level respectively, so as to leave a liquid space between the insulating sleeve and the outer electrode which will be filled with liquid when the liquid level is that high or higher. It will be understood that suitable apertures are provided through the outer electrode so that the same liquid level always exists inside and outside the probe.

It has been found that while any insulating material which is capable of use in contact with the liquid being measured and at the temperature and pressure conditions at which the device must operate is satisfactory in accordance with the present invention, a preferred material for this use is polytetrafluoroethylene (commercially available under the trademark Teflon).

The apparatus of the present invention will be better appreciated by reference to the accompanying drawings, in which:

FIG. 1 illustrates diagrammatically and principally in vertical sections a portion of a liquid hydrogen-containing tank and a liquid oxygen-containing tank as they are arranged in one of the missiles now being constructed;

FIG. 2 is an elevation of the entire probe substantailly in proportion to the dimensions of the one designed for use in the missile as aforesaid;

FIG. 3 is an enlarged View, substantially in vertical section on the line 3-3 of FIG. 2, of the lower portion of the probe of FIG. 2 showing a profiled section constructed in accordance with the present invention;

I FIG. 4 is an inverted plan view on a further enlarged scale of the probe of FIG. 3;

FIG. 5 is a view in transverse vertical section on the line 55 of FIG. 3 and on substantially the same scale as that of FIG. 4; and

FIG. 6 is a view in transverse vertical section on the line 66 of FIG. 3 and on substantially the same scale as that of FIG. 4.

As above generally set out, the capacitive probe of the present invention which is here indicated generally at 10 is arranged particularly for use in measuring the amount of liquid hydrogen in a substantially cylindrical tank 11 having as its bottom wall, a half of a spherical tank 12, which is adapted to contain oxygen in the missiles hereinabove referred to. The probe 10 may be disposed in the tank 11 in an inclined manner as shown, so that its lower portion end shown at 13 extends a substantial distance (usually as far as possible) into the rapidly narrowing space approaching the point of tangency between the cylindrical tank 11 and the spherical tank 12. Suitable brackets (not shown) are used to anchor the probe 10 in a desired position, so as to ensure that it will not be dislodged by the forces which are exerted on it incident to the take-off of the missile or to forces which might be exerted on it incident to a change of the course of the missile, which may be accomplished by means forming no part of the present invention.

In the particular embodiment of the invention here shown, the only portion of the probe which needs to be profiled or arranged so as to have a response other than can be provided by the concentric inner and outer electrodes is the lower portion shown at 13, Le. that portion which is extended below the top of the spherical bottom of the container 11 as seen in FIG. 1. Therefore, it is to the construction of this portion of the probe to which the principal part of the present description will be particularly directed. The remainder of the construction may be assumed to be conventional.

As shown best in FIGS. 3-6, the probe consists essentially of an outer tubular member or electrode 14 of some suitable metal as aluminum, wherein the tubular member 14 has a subtsantially constant inside diameter and may or may not have a substantially constant outside diameter. In some forms of the invention the outside of the electrode 14 may be provided with bracket-engaging means or means for cooperation with brackets, none of which is shown in the accompanying drawings, but which have a function solely of mechanically securing the probe in position or assisting in so doing. In any event, they are immaterial from the electrical point of view and from the point of view of the functional operation of the probe, except that the probe should be appropriately rigidly fixed in use in the container being measured.

Inside the tubular outer electrode 14 and preferably concentric therewith throughout its length is an innerelectrode 15, which may be formed either as a solid rod or as a tube, the latter construction being illustrated. This electrode should be of good electrically-conductive material, and within that limit, may be of any desired material, such, for example, as aluminum, copper, brass or the like or some suitable metallic alloy. The outside of the electrode 15 is cylindrical; and the diameter thereof is substantially constant preferably throughout its length. Thus it is not weakened by any attempt to compensate for varying cross-sectional areas of the tank by cutting the diameter thereof as aforesaid. Proportionality between the capacitive change and the amount of liquid in the tank at each level respectively is provided by other means presently to be described.

In accordance with the present invention the means for providing the desired proportionality or profiling, as it is sometimes called, comprises at least one, and in this case a single one, body of insulating material, which in the present instance is shown as a sleeve-like member 16, which has a hole centrally thereof, so that it will closely surround the inner electrode 15. It will be seen from the drawing FIG. 3 that the member 16 has progressively smaller outside diameters at stepped intervals from its lower end portion to the uppermost end thereof. This is one possible construction of the insulating member 16. In the case of this type of construction, the diameter of each portion or increment of height is calculated to be that which will leave an annular space for liquid between the outside of the insulating member 16 of the inside of the outer electrode 14 such that a rise in the liquid level of a predetermined amount, say from one of the steps shown in FIG. 3 to another, will correspond to a certain definite volume or weight of liquid in the tank. It is also known by calculation and confirmed by actual trials that the change in capacitance of the capacitor incident to the presence of liquid in this space or increment of height between the member 16 and the outer electrode 13 will vary by a definite amount, expressed, for example, as micromicrofarads. The diameter of the member 16 can then be calculated so that the desired proportionality shall exist in this small vertical increment of liquid height, so that the change in capacitance in terms of micromicrofarads is a desired and preferably a substantially constant or linear function of the amount of liquid in the tank in this vertical increment.

It will be understood further that the shape of the in'sulating member need not be actually stepped as shown in FIG. 3, but may be a smooth curve with the same limitations as aforesaid and with the curvature nicely calculated so as to attain the same mathematical result.

It will be understood that the outer electrode is provided with a sufiicient number of holes, as shown, for example, at 17, so that the height of liquid within the probe will always be substantially the same as that in the tank outside the probe.

In the present instance the insulating member 16 serves a dual function, in that the lower end thereof, shown at 18, has substantially the same outside diameter as the inside diameter of the outer electrode 14, so as to have a substantially tight fit therewith. These parts 14 and 16 may be secured together by forming this lower end 18 of the member 16 with a reentrant depression 19 to leave an annular flange 20 adjacent to the outer electrode. Suitable rivets 21 or other securing devices as nuts and bolts may then be provided as shown in FIGS. 3 and 4 for securing the insulating member 16 firmly in the outer electrode 14. The inner electrode 15 as above set forth extends through a bore centrally of the insulating member 16 which is of approximately the same diameter, so that a reasonable tight fit around the inner electrode is assured. However, in order that the inner electrode shall not move axially of the insulating member by inadvertence or the like, the lower end of the inner electrode, which is shown as a hollow tubular member in the present drawings, is bored for the reception of a transverse pin 22 which extends through aligned transverse bore holes in the lower end portion of the insulating member 16, stopping short of the outer electrode 14, so that no short circuit shall exist.

At one or more levels above the bottom of the probe it may also be desired to assure the accurate coaxial relationship between the inner and outer electrodes 15 and 14. In the event that this is needed intermediate the upper and lower ends of the insulating member 16, it may be accomplished by means as shown best in FIG. 6 comprising a plurality of insulating plug members 23 having smaller diameter inner or stud portions 24 extending into radial bores in the insulating member 16 and with the larger diameter portions of the plugs 23 extending between the outside of the insulating member 16 and the inside of the outer electrode 14. It has been found that such insulating plugs do not substantially affect the response of the capacitive probe, at least when they are not repeated too frequently throughout its length, which is the usual case. Above the top of the insulating member 16 as shown, any conventional means (not shown) such as those heretofore in general use, may be employed for assuring that the inner and outer electrodes shall remain concentric with one another throughout.

By the use of apparatus in accordance with this invention it has been found that the desired proportioning or profiling of the electrical response of the probe to situations far out of the ordinary may be provided, so that a given capacitance change will always correspond to a definite change in liquid capacity, whether that capacity be expressed as weight or volume.

Thus, in the case of hydrogen and oxygen where the present device is to measure hydrogen, it is known that the dielectric constant of liquid hydrogen is about 1.25. It is also known that the density of liquid hydrogen changes approximately linearly with its dielectric constant. From these facts it can be calculated what constants must be embodied in the circuitry to be associated with the probe of the present invention so that a resulting output of the system is substantially accurately indicative of the weight of hydrogen in the tank at any instant. When similar data is available as to oxygen, which is normally employed in the spherical tank shown at 12 in FIG. 1, these two elements of data can be simultaneously supplied to a computer, which controls the rate of use of hydrogen and oxygen during the flight of a missile under the influence of the force generated by the combustion of these two elements. Furthermore, when these two gases are about used up, this fact can be made apparent to the computer by use of the probe of the present invention, so that the section of the missile housing the hydrogen and oxygen containers may be separated from the section or sections thereof which are to proceed further, without loss of any fuel and yet without the drag which would be occasioned if the separation of the spent fuel section of the missile were to be carried along with the remainder thereof for a distance substantially beyond the time when its fuel was exhausted.

As stated hereinabove it is necessary that all the parts making up the probe of the present invention shall be capable of use in contact with the type of liquid with which they are to be used under the existing or expected conditions of temperature and pressure. Thus there must be zero or an irreducible minimum of chemical reaction between such liquid and the material used as an insulating material. In the present instance and for use in conjunction with liquid hydrogen, it is found that polytetrafluoroethylene (commercially sold under the trademark Tefion) is satisfactory and also that the metals hereinabove mentioned are usable. One particular advantage of this type of insulating material is that while it is hard and mechanically desirable in its mechanical relationships in this structure as a centering means for the inner electrode, it is also quite inert chemically and further, it can be relatively easily machined to vary its diameter to fit the mathematical pattern worked out in advance for the particular tank or use in which it is to serve.

While there is herein shown and described but one specific embodiment of the present invention, various equivalents have been described or generally referred to as the description has proceeded. It is contemplated that other equivalents will occur to those skilled in the art from the foregoing disclosure. I do not wish to be limited, therefore, except by the scope of the appended claims, which are to be construed validly as broadly as the state of the art permits.

What is claimed is:

1. A capacitive probe adapted to be fixedly mounted in a container having different liquid-containing cross-sections at different levels, for sensing the amount of liquid in the container, and to have a response in change of capacitance which is substantially linearly proportional to changes in the liquid contents of the container even throughout portions where very small changes in liquid contents result in very substantial changes in liquid level, comprising an outer electrically conductive tubular electrode of substantially constant inside diameter along its length, an inner electrically conductive electrode having along its length an outer substantially cylindrical surface of substantially uniform diameter less than said inside diameter, an elongated annular member of insulating material positioned between said electrodes and having along its length transverse cross-sectional dimensions which provide a liquid receivable free area between said annular member and at least one of said electrodes and a value of capacitance between said electrodes bearing a substantially constant functional relationship to the liquid content of the container within which the probe is mounted, and means for supporting said electrodes and member in coaxial insulated relation while admitting liquid from the container into said free area between said electrodes and said member to a liquid level corresponding to that within the container.

2. A capacitive probe in accordance with claim 1 for use in the measurement of cryogenic liquids, wherein said insulating material consists essentially of polytetrafluoroethylene.

3. A capacitive probe in accordance with claim 1 for measuring liquid contained in a substantially cylindrical chamber having its axis substantially vertical and a bottom which is reentrant inwardly thereof and is formed substantially as half a sphere having the same radius as that of said cylindrical chamber and which sphere is substantially tangent with said cylindrical chamber, wherein said annular member has cross-sectional dimensions progressively decreasing along its length from a large crosssectional area at its lower end to a small cross-sectional area at its upper end.

4. A capacitive probe in accordance with claim 3, wherein said annular member is fixed at its lower end to the outer electrode and has a bore through which said inner electrode extends, the diameter of said bore approximating the outside diameter of said inner electrode, and which includes means for anchoring said inner electrode within said annular memberadjacent to the lower end thereof.

5. A capacitive probe in accordance with claim 4 in which said annular member extends a substantial distance longitudinally of said probe, and in which said support means includes electrically insulating spacers fixedly disposed at intervals along said probe and extending between the outside of said annular member and the inside diameter of said outer electrode, so that said annular member and said inner electrode will be maintained concentric with the inside diameter of said outer electrode throughout their lengths.

6. A capacitive probe in accordance with claim 1, in which said annular member is in the form of an elongate annular sleeve surrounding and closely embracing said inner electrode and having a maximum outer diameter less than the inner diameter of said outer electrode so as to leave an annular space between the outer surface of said annular member and the inside diameter of said outer electrode but additionally having along its length differing outer diameters selected to have an inverse proportional relation to the cross-sectional area of the liquid in the container in which said probe is located so that a given change in the electrical capacitance of said probe will correspond to a predetermined increment of liquid contents of the container throughout the portion of the container wherein the liquid is measured by said probe.

References Cited by the Examiner UNITED STATES PATENTS 1/52 Smith 317-246 X 2/55 Rickner 317-246 OTHER REFERENCES JOHN F. BURNS, Primary Examiner.

JOHN P. WILDMAN, Examiner. 

1. A CAPACITIVE PROBE ADAPTED TO BE FIXEDLY MOUNTED IN A CONTAINER HAVING DIFFERENT LIQUID-CONTAINING CROSS-SECTIONS AT DIFFERENT LEVELS, FOR SENSING THE AMOUNT OF LIQUID IN THE CONTAINER, AND TO HAVE A RESPONSE IN CHANGE OF CAPACITANCE WHICH IS SUBSTNATIALLY LINEARLY PROPORTIONAL TO CHANGES IN THE LIQUID CONTENTS OF THE CONTAINER EVEN THROUGHOUT PORTIONS WHERE VERY SAMLL CHANGES IN LQUID CONTENTS RESULT IN VERY SUBSTNATIAL CHANGES IN LIQUID LEVEL, COMPRISING AN OUTER ELECTRICALLY CONDUCTIVE TUBULAR ELECTRODE OF SUBSTANTIALLY CONSTANT INSIDE DIAMETER ALONG ITS LENGTH, AN INNER ELECTRICALLY CONDUCTIVE ELECTRODE HAVING ALONG ITS LENGTH AN OUTER SUBSTNATILLY CYLINDRICAL SURFACE OF SUBSTANTIALLY UNIFORM DIAMETER LESS THAN SAID INSIDE DIAMETER, AN ELONGATED ANNULAR MEMBER OF INSULATING MATERIAL POSITIONED BETWEEN SAID ELECTRODES AND HAVING ALONG ITS LENGTH TRANSVERSE CROSS-SECTIONAL DIMENSIONS WHICH PROVIDE A LIQUID RECEIVABLE FREE AREA BETWEEN SAID ANNULAR MEMBER AND AT LEAST ONE OF SAID ELECTRODES AND A VALUE OF CAPACITANCE BETWEEN SAID ELECTRODES BEARING A SUBSTANTIALLY CONSTANT FUNCTIONAL RELATIONSHIP TO THE LIQUID CONTENT OF THE CONTAINER WITHIN WHICH THE PROBE IS MOUNTED, AND MEANS FOR SUPPORTING SAID ELECTRODES AND MEMBER IN COAXIAL INSULATED RELATION WHILE ADMITTING LIQUID FROM THE CONTAINER INTO SAID FREE AREA BETWEEN SAID ELECTRODES AND SAID MEMBER TO A LIQUID LEVEL CORRESPONDING TO THAT WITHIN THE CONTAINER. 